Genetic analysis relies on the ability to introduce, eliminate or modify genes at will. Such techniques are advanced in genetic model organisms such as yeast, flies and mice, but are limited in the worm C. elegans, which is one of the most commonly used model organisms. In worms such methods are somewhat crude: gene knockouts are by random mutagenesis and restoring gene function is via multicopy extrachromosomal arrays or random gene integrations. Despite these limitations C. elegans research has contributed to some of the most important discoveries in biology in the last 35 years: Ras - MAP kinase pathways, cell death pathways, RNA interference and posttranscriptional regulation by microRNAs are examples. We propose to develop techniques to insert, delete or modify genes in the C. elegans genome. The techniques rely on mobilizing transposons to engineer the genome; the methods will be a resource for the whole C. elegans research community. Aim 1. Improved single copy insertion. We will characterize insertion sites on each chromosome and increase the efficiency of transgene insertions. We will also devise transient selection reagents that can be used in a wild-type background. Aim 2. Universal insertion sites. We will generate universal insertion sites on all chromosomes that will be compatible with a single targeting plasmid. This will substantially increase the versatility of the technique. Aim 3. Gene targeting. We will develop a strategy to manipulate genes in their endogenous context. This technique will allow researchers to engineer mutations, including knock-outs, without any extraneous DNA changes in the gene except the intended mutation.